1. Field of the Invention
The invention relates to a process for the production of polyphenylene ethers which have a low content of components which absorb visible light. More specifically, it relates to the production of polyphenylene ethers by the oxidative coupling of phenols in the presence of metal amine complexes, followed by the addition of boron containing reducing agents.
2. Discussion of the Background
Polyphenylene ethers represent a family of technically valuable synthetic materials, particularly in combination with impact-resistant modified polystyrene (see, e.g., U.S. Pat. No. 3,306,875).
Polyphenylene ethers are usually produced by the oxidative coupling of monovalent phenols in the presence of catalysts, such as metal amine complexes, particularly copper amine complexes in solution. Preferred solvents are aromatic hydrocarbons, such as toluene, optionally in a mixture with aliphatic alcohols, such as methanol. At the end of th self-condensation, the reaction mixture contains the desired polyphenylene ether, the catalyst complex, nonconverted phenol, oligomers and colored by-products, such as tetramethyldibenzoquinone or dimethylbenzoquinone (see U.S. Pat. No. 4,060,514, column 2, lines 25-34). The desired polyphenylene ethers also obviously contain visible light absorbing chromophores (see Macromol. Chem. 180, 2875-82 (1979)).
The reaction is usually terminated by inactivation of the catalyst. This is done, for example, by the addition of aqueous solutions of inorganic or organic acids as described in DE-OS 21 05 372. Also used are polyaminocarboxylic acids (see DE-OS 23 64 319) or other chelating agents, such as tri(carboxymethyl)amine, its sodium salts or ethylenediaminetetraacetic acid and its sodium salts (see DE-OS 25 32 477). The latter may also be in combination with quaternary ammonium salts (see U.S. Pat. No. 4,026,870). Apart from terminating the oxidative self-condensation, the addition of complexing agents also provides for the most complete possible removal of the catalyst from the polyphenylene ether, as this type of contamination leads to a deterioration of the overall properties of the polymers.
If direct isolation methods (see DE-OS 33 37 629 and EP-PS 0 126 972) are used for the isolation of polyphenylene ethers, it is usually necessary to employ additional measures in order to stabilize the polyphenylene ether solutions against a molecular weight loss and to reduce the content of colored by-products. In DE-OS 26 16 746 it is suggested to treat the polymer solution with reducing agents, such as sodium dithionite or hydrazine. However, in most cases this measure is not sufficient to achieve stabilization for long periods of time. In DE-OS 24 30 130 and OS 27 54 887 and U.S. Pat. No. 4,060,514 improved procedures are described, whereby, apart from the reducing agents, additional bivalent phenols or phase transfer catalysts, such as quatenary ammonium salts are used. Advantageous procedures are also decribed in DE-OS 33 32 377 and OS 34 42 141.
All these procedures have the disadvantage that, while they reduce the low-molecular weight contaminants and the polymer bound colored chromophores, they do not provide for irreversible removal or blockage. Thus, the colored byproducts can be reformed after the use or destruction of the reducing agent. This is the case in particular for colored defects built into the polymer chain. It is not sufficient to adsorptively remove the colored low-molecular weight contaminants for the production of usable molding compounds on the basis of polyphenylene ethers as described in U.S. Pat. No. 4,391,950, Example 3. Only the addition of benzoic acid anhydride (U.S. Pat. No. 4,391,950, column 5, lines 46-57) produces usable molding compounds which are stable at temperatures of 300.degree. C. However, this is based on a methanol precipitated polyphenylene ether powder (see example 2) or crumbs (by hot water crumbling, see example 1). In direct isolation procedures, as described, for example, in DE-OS 33 37 629 or EP-PS 0 126 972, it is shown, however, that these measures are also not sufficient.